The present invention relates to rotary kilns, and more particularly to a toothed grate for covering the peripheral discharge opening of a rotary kiln which utilizes satellite or planetary coolers.
A conventional rotary kiln comprises a simple cylinder or shell installed at an incline. Raw material feed to be pyroprocessed is charged into the kiln from an inlet at the elevated end, and thereafter the feed material is typically calcined by applying heat while the cylinder is rotated, and the calcined product is discharged from an outlet at the exit or lower end of the kiln. Kilns of this type are well known in the art and are capable of treating raw materials of many different kinds and a wide variety of particle sizes and shapes.
The hot pyroprocessed material exiting from the kiln is then typically cooled before further processing. Many different types of arrangements have been developed for cooling such material. One such arrangement is referred to as planetary or satellite coolers arranged circumferentially about the outside of the rotary kiln. When passing from the kiln to a peripheral cooler, pyroprocessed material exits the kiln via a plurality of circumferentially spaced peripheral discharge openings which in the past have been covered by a grate mounted horizontally within the discharge openings. The openings in the grates are sized to permit passage of a normal size range of product to the coolers and to reject oversized particles which could pass into the cooling unit and potentially act to plug the coolers and render them inoperable. In addition, the grates assure a quality product by segregating out the oversized particulate material which would otherwise render the product nonhomogeneous.
Although the use of grates has numerous advantages, as referred to above, grates also have numerous disadvantages. One disadvantage is that a grate which is located within the discharge opening actually reduces the area of the opening thus resulting in an increase in air velocity from the satellite coolers into the kiln than would be normal without the grates in the discharge openings. This relatively high velocity stream of air picks up or entrains fine material and carries it back into the interior of the kiln rather than allowing such material to discharge or exit from the kiln. Another disadvantage of grates that are located within the discharge openings is that since the grate reduces the area of the discharge opening, the rate of material discharge is also limited or reduced.
It is thus desirable to provide an arrangement at the discharge openings of a rotary kiln utilizing satellite or planetary coolers that would reduce or minimize the possibility of a flowback of fine material into the kiln as well as reduce or minimize the resistance to material discharge from the kiln caused by prior grates.
The present invention provides a raised grate disposed over each peripheral discharge opening of a rotary kiln that lead to peripheral or satellite coolers. The grate has a generally convex geometric profile that projects into the interior of the kiln, and is not in the plane of the peripheral discharge opening itself, as is prior art grates. The generally convex geometric profile or shape of the raised grate provides a greater open flow area than is provided by prior flat grates which were located in the plane of the peripheral discharge opening. The increased open areas available in the raised grate of the present invention are utilized in a counter flow fashion for the discharge of material from the kiln to peripheral or satellite coolers, and for the passage of cooling air exiting the peripheral or satellite coolers and entering the kiln via the discharge openings. Raising the grate so that it projects into the interior of the kiln increases the available area for countercurrent material and gas flow. This increase in available flow area of the discharge opening by not having the grate in the plane of the discharge opening, as well as the increased open area provided by the raised grate itself, reduces the velocity of the air entering the kiln from the tube cooler which in turn results in reduced dust entrainment in the air stream and therefore a reduced recirculating dust load within the interior of the kiln.
The edges of the grate that face the interior of the kiln are also lined with teeth. The rotary action of the kiln will bring the grate teeth into contact with agglomerated dust balls that form within the kiln. The repeated contact between the grate teeth and the dust balls that are too big to pass through the openings in the grate, will break pieces off the dust ball that are small enough to pass through the openings in the grate and enter the peripheral coolers. Thus, a relatively large dust ball is ultimately broken down such that it will completely pass through the grate and enter the peripheral cooler. The use of such teeth will eliminate the need to follow the prior art wherein a ring of lump breaker castings are installed at the discharge end of the kiln. In addition, the prior art lump breaker castings can be replaced by a refractory discharge dam at the exit end of the kiln that will keep the dust balls in contact with the toothed raised grate and directs the crumbled dust ball to pass through the openings in the tooth raised grate, through the discharged opening in the kiln and into the peripheral coolers along with the main product stream. Prior art lump breaker castings on the kiln exit end have the disadvantage of sometimes allowing crumbled dust balls to discharge from the kiln end and thereby bypassing the peripheral coolers and being separated from the main product stream. The use of a toothed raised grate together with a discharge dam will minimize any product bypassing the main product stream and will assure a quality product of substantially similar sized particulate material.
In accordance with the present invention, the toothed raised grate comprises a grid portion, a plurality of feet spaced from and extending substantially parallel to the grid portion, and a plurality of legs interconnecting the grid portion and feet. The grid portion, feet and legs are integrally formed of a plurality of spaced lengthwise extending bars and a plurality of crosswise extending bars so as to have a generally convex or xe2x80x9cspider-likexe2x80x9d geometric profile. Both the grid portion and legs preferably include teeth projecting therefrom.